With carrier aggregation, multiple carriers are used together, to provide service to a node, e.g., a wireless communication device. That is, two or more separate radio carriers are “collected” together in some functional or operational sense. The manner of aggregation and the underlying operational details vary according to the type of radio network and/or services involved. In networks based on LTE or LTE-Advanced, which is a progression of the Long Term Evolution (LTE) standard, carrier aggregation increases the available transmission bandwidth.
The bandwidth increase comes from using multiple carriers concurrently for data transmission, which effectively increases the transmission bandwidth in LTE beyond the maximum single-carrier bandwidth of 20 MHz. The multiple carriers may be contiguous within a larger spectrum, or they may be non-contiguous, where component carriers from different spectrum bands are aggregated. The former case is referred to as “intra-band” aggregation while the latter case is referred to as “inter-band” aggregation.
The “flexible cell” concept introduced by ERICSSON AB provides another example of carrier aggregation. The concept relies on “soft-cell” operation in a heterogeneous (“HetNet”) network context where a wider-area, macro coverage area overlays one or more smaller coverage areas provided by respective low-power nodes. The low-power nodes can be understood as improving radio coverage and/or providing the opportunity for higher-rate data service for devices operating within their coverage areas.
In some HetNet deployments, a macro node provides a macro cell that overlays one or more low-power cells provided by corresponding low-power nodes positioned within the macro coverage area. The macro and low-power cells operate as separate cells, each associated with a unique cell identifier within the network. Contrastingly, in a HetNet configured according to the soft-cell concept, a low-power cell and its overlaid macro cell share the same cell identifier. This arrangement exploits the difference between a “cell” and a “transmission point.”
Cell-specific references signals (CRS) functionally depend on unique cell identifiers within the network, and allow devices operating within the network to demodulate cell-specific control and synchronization information, as needed for network access. In contrast, a transmission point simply represents an antenna or antenna element from which a device can receive data transmissions within the “cell.” Demodulation-specific Reference Signals (DMRS) transmitted from each data transmission point within the cell enables the receiving device to determine the channel and precoding associated with the transmission.
Thus, in the soft-cell context, the macro cell and the low-power cell function as a “shared” cell, with the macro node and the low-power node representing different transmission points within the shared cell. This arrangement allows, for example, a division between the transmission of certain system information and user-plane data. Namely, the macro layer provided by the macro node is used for broadcasting certain system information to devices operating within the broader coverage area represented by the shared cell, while the low-power layer provided by the low-power node is used for high-rate data service to one or more devices operating within the low-power coverage area overlaid by the macro-layer coverage area. For further soft-cell details, see “Heterogeneous network deployments in LTE,” Parkvall, et al., Ericsson Review (February 2011), which discusses the use of frequency division for resource partitioning and Almost Blank Subframe (ABS) signaling, for devices having multi-frequency carrier aggregation capabilities.
The above soft-cell arrangement decouples system information and the control plane from user data. In the simplest implementation, control plane transmissions by the macro node and user data transmissions by the low-power node use the same carrier frequency and are “separated” in the soft-cell context only in terms of which node is involved in the transmission. With spectrum optimization, however, the control information is transmitted in a first frequency band by the macro node, and user data is transmitted in a second frequency band by the low-power node(s). Example frequency bands used in cellular radio networks include the 800 MHz, 900 MHz, 1800 MHz, and 1900 MHz bands.
Spectrum optimization comes at the expense of complexity, as the use of different frequency bands obligates devices within the soft-cell to maintain concurrent communications in both frequency bands. The requirement to maintain concurrent communications in multiple frequency bands obligates the device to have additional or more complex radio transceiver circuitry. This requirement harmonizes with the general trend toward the use of carrier aggregation, which necessarily requires compatible devices to support concurrent operations across more than one frequency band, but it presents problems in the case of “legacy” devices that were designed for operation in only one frequency band at a time.
A known accommodation addresses the legacy-device problem with the introduction of measurement gaps in the data transmissions. These gaps, e.g., 6 ms gaps every 40-100 ms, are not useful for data transmissions, especially because of the attendant retransmission timing and latency issues, but they do allow legacy devices time to monitor the macro-layer for small amounts of control signaling. Such monitoring interrupts data transmission on the low-power layer, however. With gap periodicity ranging from 40-100 ms, the use of measurement gaps meaningfully reduces data throughput on the low-power layer.